[SI-LIST] Re: Am I missing something on CM Impedance vs Diff Impedance

Perhaps the ferrite on the external cable is attenuating common mode noise
present on the ground or return lines, whereas the bead-on-lead ferrites are
only affecting the signal lines. In addition, a poor RF connection of the
ground lines to the board and chassis can make problems worse.

Joel
  

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Erin.McPhalen@xxxxxxxxxxxxxxxxxxxxxxxxx
Sent: Friday, April 18, 2008 11:44 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Am I missing something on CM Impedance vs Diff Impedance

 
Hi
During testing at a EMC lab, the solution to a radiated emissions problem
wasfound using a standard cable clamp-on type ferrite choke (MAT28) over a
cable(no shield).  In order to move the solution to the inside of the
product, a series of bead-on-lead type ferrites (multi turn, same material) 
where used on each line.   

The overall reduction in emissions at the frequency in question was
significantly less using the one per line bead-on-leads type compared to the
clamp-on common mode choke, even though the impedance at the frequency in
question was substantially higher(x4) with the bead-on-lead ferrite. 

My question is, why would a Common Mode choke with say 200 Ohms at 100 Mhz
over all signal lines perform better than  individual chokes on every signal
line with an impedance of 800 Ohms at 100 MHz. 

Thanks in advance, 

Erin
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